Monday, April 27, 2009

Ghost-hunting at Pylos

Imagine yourself in the countryside during a clear night staring at the countless stars that never failed to inspire poets, science fiction writers and lovers. For those lucky enough to have viewed the sky through a powerful telescope, the splendid beauty and immensity of the cosmos acquire near-religious potency. Astronomers, a race of stellar priests-cum-poets of sorts, use electromagnetic radiation – such as light or radio waves – that is emitted by those distant worlds in order to investigate the mysteries of the universe and to explore the evolution of stars and planets. However, there are parts of the universe that are opaque to optical or radio telescopy. For example the nuclei of stars, or the dense centers of galaxies, or the early universe when light did not yet exist.The experimental confirmation of the mysterious neutrino particle in 1956 ushered a new era in astronomy - called “neutrino astronomy” - that can boldly go where other astronomies cannot, and see things no optical or radio telescope can.Neutrinos (not to be confused with neutrons) were first theorized to exist in 1930 by the famous physicist Wolfgang Pauli. They are tiny particles which result from nuclear reactions, such as the ones taking place in the Sun or inside a nuclear reactor. Unlike electrons (which are negatively charged) neutrinos have no electric charge and are virtually massless. Thus they pass through matter virtually unhindered. Every second 50 trillion of those ghostly things pass through your body without you noticing it!The bonus of being like a ghost is that you can travel through anything, anywhere, anytime. Compare that with photons, the particles of light. Photons produced inside the core of the Sun take 40,000 years to reach its outer surface and become visible by our optical telescopes. This is because photons interact with the electromagnetic forces inside the core of the Sun which impede their transmission. Neutrinos, having no charge and interacting with nothing, travel from the core to the surface almost instantly. A similar phenomenon occurs during the final stages in the lifetime of a big star. When such a massive star burns out it implodes and then violently explodes all its matter and energy into space. The phenomenon is called supernova and is one of the most spectacular and amazing events in the universe. In 1987 detection of a massive “neutrino storm” foretold a supernova explosion 18 hours before the light from the explosion arrived at Earth.Let us return for a moment back to our imaginary telescope. Modern astronomy tells us that all the stars and the galaxies out there, all “ordinary matter” as it is called, accounts only for 4% of the universe. Of the rest, 22% is made up of the mysterious dark matter, and 74% of the even more mysterious dark energy. Dark matter is responsible for the way galaxies are clustered together, it gives the universe its “shape” and it is made up from weakly interacting particles of matter that are still unknown. Neutrinos may account at least for a part of the “missing” dark matter.

With so many cosmic mysteries to solve, the scientific interest in neutrinos is great and since the late fifties many scientists and engineers have tried to develop machines that could detect them. This is no easy feat. For example, if we wanted to block half the typical neutrinos that emanate from our Sun we would need a sphere of water around the sun with a radius of 10 light years!

The coast of Pylos may not hold so much water but it does hold enough for Nestor, an experimental neutrino telescope. The design for Nestor is based on a rigid metal structure that supports arrays of thousands of sensors which detect the faint collisions of neutrinos as they pass through Earth. The structure, more than 10 times higher than the Eiffel Tower, has been immersed in the deep waters outside Pylos, where depths go down to 5 kilometers. So far only 2 of the total 12 floors of the structure have been assembled. The sea water is important because it absorbs most of the light as well as other electromagnetic radiation, allowing only the ever-elusive neutrinos to pass through.

In fact it is the great depth, combined with the close proximity to the shore that Pylos is putting forward as its comparative advantages in competing with the two other European observatories, Antares in France and Nemo in Italy. The prize will be the selected site for KM3Net, the future European infrastructure for neutrino telescopy. At the southern hemisphere, on icy Antarctica, another neutrino telescope – aptly called IceCube - is being built already. Together, IceCube and KM3NeT will view the full sky while searching for neutrino sources, such as gamma ray bursts, supernovae or colliding stars. A final decision for KM3Net is expected within the next two years and the Greek scientists, as well as the Greek Government, are trying to convince their European counterparts for the scientific as well as geographical values of Pylos. Till then, the existing detectors of Nestor will keep on searching for the ethereal passes of the tiny neutrinos, listening deep into the dark waters of the Mediterranean Sea, perhaps the most unlikely and curious place to watch the sky.